Geochemical prospecting



Feb. 1.9, 1946. E E, RQPER 2,395,014

GEOCHEMICAL PROSPECTING Filed July 30. 1942 2 Sheets-Sheet 1 5cm/en? J'rzvefz for.. [du/fz E. Roper..

TRAP

' one instance a value of 90 of only 14 parts per reason for this decrease in apparent hydrocarbon'.

Patented 419,194.6

l 2,395,014 GEocnEimcAL Pnosrnc'rmov Edwin E. Roper, Tulsa, Okla., `assigner to Stanof` lind Oil and Gas Company, Tulsa, Okla., .a corporation of Delaware Application July 30,

7 Claims.

neath the ground and more particularly -to a process for analyzing certain components to be a found in the soil whereby itis possible to determine the approximate location of said subterranean petroliferous deposits.

It is .known that the soil itself at or near the surface of the earth and also the air in this `soil contains various substances which are indicative of the presence of petroliferous deposits below.

Such indicator substances may be gaseous hydrocarbons;l liquid hydrocarbons and solid or waxy hydrocarbons and other materials. I t. is furthermore known that the quantity of the indicator substances is a measure of the prob,-

ability o1' existance of a petroleum deposit below.v These substances have been takenfrom the soil' and also from the air in the soil and analyzed by various means to obtain a value which can be placed on a graph or map or other type of chart. When a series of such values have been obtained, the map or chart will exhibit certain characteristics which can be utilized-for locating oil 'deposits.

Often the indicator substance is contained in the soil gases. By the term soil gases I intend torefer to both the air obtained 'from the inter.. stices oi the soil as well as the gases obtained by desorption of soil samples. The soil gases are treated in a purification train or the-like so as to remove certain contaminating gases such as CO2,

water vapor and so forth, and the quantity of uncontaminated hydrocarbon gases is then measured to obtain a value which can be recorded as mentioned above.

It has been found,however, that the concentration of apparent hydrocarbon gases derived from a single sample of soil gas varies over a period oi time, decreasing at first comparatively rapidly and then more slowly. For example, in

parts per million of apparent hydrocarbon was obtained w en the gas was analyzed immediately after, the from the soil sample. The same gas, after standing for a periodo! about. 90 hours, gave a value million.,

content is xiotftool clear but it isrbelieved, for example when the soil samples are desorbedf. by

cur andthat certain spontaneous reactionsmay the evolved gas during the ensuing period which'decrease'the measurable quantity or concentration of apparent hydrocarbons. 5

desorption The exact cause or v decompositions ocv'1942, sensing. 452,842 4(ci. 23e-232) If the gases, after desorption, are permitted to stand for any length of time these reactions proceed and thereby affect the measurableapparent hydrocarbon content of the gas when itis analyzed.

5 I have found'that a substantially constant value of true hydrocarbon concentration can be obtained by treating the apparent hydrocarbon gases, after desorption, with a strong oxidizing agent. While the gasesl may be treated very shortly after desorption, it is not essential that this step be carried out immediately thereafter for substantially constant values may be obtained even though the treating step is carried out some time after desorption. When the gases are so treated and subsequently analyzed for hydrocaranalyzed immediately following the desorption and treatment'with oxidizing agent or whether a -period of several hours or days is permitted to elapsebefore the final analysis. vIt is believed that the treatment with the oxidizing agent removes from the gaseous mixture certain unstable or very reactive compounds, which. if permitted to remain vin the gaseous mixture spontaneously react over a period of time. It has been noted that the-values obtained by following the method of the present invention are in general substantially the same as those obtained by analyzing soil gases whichhave been.permitted to stand for extended per-logsl during which time the V:spontaneous reactions occur. It would seem that according to known processes the unstable compounds, which are formed by the desorption step,

would increase the apparent value of the hydrocarbon concentration, whereas if the hydrocarbon mixture containing the unstable lcompounds is Permitted to stand, the latter will react to form various compounds whichwould be removed from the hydrocarbon stream before the ilnal analysis is made. When air obtained from the interstices ofthe soil is used asa. prospecting method, substances are present which are stable but'are not hydrocarbons; the method claimedin myinvention removes, these 4spurious substances and gives actual hydrocarbon values.

An object of theinvention is to provide animprovedvprocess for analyzing soil'gas'esobtained by desorption of soil samples to obtain more re jliable information with' minimum delay.

- improved process for analyzing soil gases obtained from the interstices oi' soll samples.

Stili another object of the present inventionis Another object of the invention isfto provide an e to provide'an apparatus for carrying out the prestroduced through line i.

1 contained therein.

. ti n '0g molal lto about 0.01 molal whereas `the KOI- may be ab`out0.1 molal. Other concentrations and vconnected to line entprocess.

Other objects of the present invention will be apparent as the description thereof proceeds.

Particular referenceis made to the drawings, forming a part of this specification in which,

Figure 1 shows a schematic ilow sheet for removing the soil gases from a soil sample according to the present invention, and

Figure 2 shows a schematic flow sheet for analyzing the soil gases.

In Figure 1 the apparatus forremoving the soil gases from av soil sample by desorption is shown. This apparatus is initially urged with purified nitrogen or other inert gas which is in- A pressure regulator 2 can be set to determine the rate of flow of the inert gas. After the system has been entirely purged, stopcock 3 is turned to establish com- 'munication between the water supply reservoir 4 and ilask (of knownvolume) and the stopcock f6 is `turned to establish communication between the ask 5 and vline l. VThe ilask 5 is then lled with water from reservoir 4. Stopcock 6 is then turned to establish communication between line 8 and flask The inert gas is introduced through line l to maintained at a temperatura` of from about 296 F. to about -320 F. by immersion for instance in liquid oxygen (B. P.'296 F.) liquid air (B. P. 306 F.) liquid nitrogen (B. P. -320 FJ, or liquid nitrogen boiling under reduced pressure. The inert gas passes from trap 9 through line t to flask t displacing the water This enables one to obtain a denite predetermined quantity of nitrogen to be used for assisting in removal oi' the hydrocarbon gases evolved from the so-il sample. Subsequently, stopcock t is turned to establish communication between ask 5 and line l.. The-inert gas innask 5 is forced through line 'l by illling ask 5 with water from reservoir d and then through now indicator i@ and line ii into the tube i2, which ls maintained at approximately 200 F. toi 212 F. by means oi a hot water bath or the li e. tube i2 carries with it the carbon gases as well as all other evolved gases which were originally present in the soil sample together with any gases formed by pyrolytic decomposition occurring in i2. The mixture of evolved gases and inert gas-leaving the tube l2 passes through line it to a scrubber ifi which contains a strong oxidizing agent. The oxidizing agent may be ins the form of an acid or alkaline solution, or in solid powder form. It has been Ifound. in this connection that KMnOt is particularly suitable for the purposes ci the present invention but other oxidizing agents such as KzCrzOvand the like may be used. If, vfor instance a KOI-I-KMnO4 solution is used in the scrubber. not only are the pyrolytic decomposition products modiiied but also the CO2 present in 'the gas mixture is partially or entirely removed. It has been found that the KMnO4 solumay have a concentration of from about evolved desorbed hydromay be used but these give satisfactory results. The puried gases pass through line l5 to flask i6 which. has been illled previously with water orlother inert liquid. When the water in flask' l0 has vbeen replaced bythe soil gases,l the ilaskI is-rer'noved from thev system shown in Figure l i8 shown in Figure Ait trap 9 which is' The inert gas flowing upward through the all times during the desorption of the hydrocarbon gases from the soil sample, a slight excess pressure of inert gas is maintained in the system i to avoid the possibility of contamination of the since the ambient temperature `is known,

in flask 2b. Stopcock v 22 is cooled to a which is to be analyzed,

gas sample by atmospheric constituents.

Since the hydrocarbon gases in admixture with the inert gas have been partially freed from the contaminating gases through the treatment with the oxidizing agent, the flask I6, if properly sealed at its outlets, may be stored -for` any length of time or may be' analyzed immediately. The ask i6 is connected to scrubber i9 (Figure 2) which in turn is connected to line 2li. Line 20 leads into the purification train having sections 2l and 22. The whole system, shown in Figure 2 from 34 to 20, is evacuated to a low pressure, of about 0.1 micron or less'. In order to equilibrlate the puriiication train with gas of a concentration o substantially the same as that subsequently to be analyzed, a certain amount, say 50 cc. to 500 cc., of the gaseous mixture in flask it is drawn through the purification train by means oi a vacuum applied` at 23 and thereby voided, stopcock 24 being closed during this step. At this point stopcock 25 is closed and sto-pco'ck 2li is opened whereby the required volume of the gas is drawn from flask it through line It, scrubber i9, line 2d, sections 2i and 22 of the purification train, into flask 26, the volume of ask 25 being known-from a previous standardization. Stopcock 2t is then closed. Since the pressure within flask 25 can be measured by manometer 2l, and the volume of the moisture-free gas within flask 2E,

may be computed. 'Section 2i of the purification train contains slum hydroxide or the like for removing any carbon dioxide present in the gaseous mixture. Section 22 of the puriilcation train contains pliesphorous pentoxide or the like for removing the final tracesmf Water vapor. As previously stated the strong oxidizing agent, such as potassium hydroxide-potassium permanganate solution con`1 tained in scrubber it, will effectively remove many compounds which might be present in the gas mixture, such as nitrous oxide, aldehydes, ketones, carbonyl oxysulde, alcohols, mercaptans, ketenes, acrolein, ,alkenes` and alkines and A mercury cut-oil 23, 29, 3d and 3i is provided so Ythat thorough pre-evacuation can be performed when the mercury level is below the-branch of the V, yet when it is above the branch of the Y, all of the measured gas sample from as'k 2t must pass through trap 3i. By opening stopcock 25,'with mercury levels at the properv amount of the puried anhydrous gas sample in ilask 26 is pumped rapidly through the trap 32 which is maintained at a temper ature of. from about l--296 F. to about -320 F. Y

or colder` by refrigerating the same with liquid oxygen, liquid air or liquid nitrogen. Vacuum is available at all times :by means of pumps not shown,l connected to 35. This processie continued until the gage at 33 shows a pressure of about 200 microns (0.2 mm.) 0r less or until only about 0.1 percent of the original sample remains 2E is then closed and the pumping continued until the condensedk material in trap 32 shows a pressure of 0.1 micron or less. This procedure removes all, oxygen, argon, nitrogen, carbon monoxide, helium, neon and the greater' part of the ethane unless the trap 320 F. or.colder, at which temperatures little ethane will remain gaseous. The coldtrap 32, containinga glass wool plug, is

potasvery emeient in its condensing action, it does not oder a -great resistance to ilow and yet retains any fog that may form.v Upon reaching y'0.1

micron the mercury level in the Y 29 isv raised up the U tubes from 29 to marks 30 and 3| thereby confining the .condensed material in trap 32 in a definite isolated volume. Atthis point the refrigerant about trapl` 32 is remo-ved and the trap is allowed to warm to room temperature. 'Ihis may be hastened, if desired, by placing a beaker of tap water about the trap. The condensed material in trap 32.vaporizes during this heating and the pressures exerted by these vapors are then ascertained on gage 33. A McLeod gage can be employed forgage 33 is desired.v A few successive readings may Ibe taken to give' assurance that the vaporization-ls complete and the final pressure obtained from the gage is the pressure of the vaporized gas-contained in the known,

xed volume 30, 32, 33 and 31. "I'he methods of calculating the Chydrocarbo `concentration when ,the above data have been' obtained `arewell known and will not be discussed herein.

It is obvious that various modifications may be used in place of those devices shown in the drawings without departing from the spirit and scope of. the present invention. It will be underj stood also that while the invention has been described with reference to a particular arrangement of apparatus, that this is not to be construed as a limitation but rather only as an example. The scope of the present invention is to be determined by the claims-.appended hereto. .1

WhatIclaim is: l. In a method of geochemical prospecting wherein-soil gases are analyzed for significant' 'removal from the soil but before analysis, with4 `a-strong oxidizing agent which is inert toward said significant hydrocarbons, said treatment being carried out under such conditions that `said impurities are largely removed from the gases, the treated gases being protected pending analysis from contamination by other hydrocarbon containing gases, whereby the subsequently obtained analytical result will be more reliable.

2. In a method loi geochemical prospecting wherein soil Agases are analyzed for signiiicant hydrocarbon content as an indication of petro- -liferous deposits below the location from which the gases were obtained, said gases containing relatively unstable impurities which 'normally'.

over a period of time,ldecreasingly exaggerate the results of the subsequent analysis for'signicant hydrocarbon content, the steps of desorbing the gases from thesoil. treating said gases, after -removal from the soil but before analysis, with an alkali metal oxidizing agent which is inert toward said significant hydrocarbons, said treatment being/carried out under such conditions that said impurities are largely removed fromA the gases, the treated gases being protected pending analysis from. contamination by vother hydrocarbon-containing gases, whereby the subsequentlyobtained analytical result will be more reliable. 3. In a method of geochemical prospectingl wherein soil gases vare analyzed for significant hydrocarbon content as an indication 4vof petroliferous deposits below the location from which the gases-were obtained, said gases containing `5 relatively unstable impurities which normally, over a period of time, decreasingly exaggerate theresults ofthe subsequent analysis for significant hydrocarbon content, the steps of desorbing the gases 'from the soil, `treating said gases, after removal from the soil but before analysis, with KMnO4, said treatment being'carried out under J such conditions that said impurities are largely removed from the gases, the treated gases being protected pending analysis from contamination by other hydrocarbon containing gases, whereby the subsequently obtained analytical result will be more reliable. l

4. In a method of geochemical prospecting wherein soil gasesl are analyzed for signicant hydrocarbon content as an indication of petroliferous deposits below the location from which the gases 4were obtained, said gases containing relatively unstable impurities which normally, over a period of time, decreasingly exaggerate the results of the subsequent analysis for significant hydrocarbon content, the steps of desorbing the gases from the soil, treating said gases, after removal from the soil but before analysis, with a KOH solution of KMnO4, said treatment being 30 carried out under such conditions that said impurities are largely removed yfrlom the gases, the treated gases being protected pending analysis from contamination by otherhydrocarbon conanalytical result will be more reliable.

5. In a method of geochemical prospecting wherein s oil gases are analyzed for significant hydrocarbon content as anindication of petroliferous deposits below the location from which the gases were obtained, said gases containing relatively unstable impurities which normally,

over a period of time, decreasingly exaggerate the results of the subsequent `analysis for significant gases from the soil, treating said gases, after removal from the soil but before analysis, with a KOH solution of KMnO4, the concentration of from a plurality-of spaced locations in the ground, f

desorbing the gases from the soil, said gases containing relatively unstable impurities which normally over a period of time decreasingly exaggerate theresults of the subsequent analysis for` significant hydrocarbon content, treating said gases,

6 after removal from the soil but before analysis.

with a strong oxidizing agent which is inerttoward said significant hydrocarbons, said treatment being carried out under such conditions that said impurities are largely removed from the l gases, protecting the treated gases pending analysis from contamination by other hydrocarboncontaining gases and analyzing the treated gases for significant hydrocarbon content for an indication'l of petroliferous deposits below the locations from which said soil samples were obtained.

taining gases, whereby the subsequently obtained hydrocarbon content, the steps o-fdesorbing the prising the steps of obtaining soil samples from.

a plurality of spaced locations in the' ground, de-

' sorbing the gases from the soil, said gases containing relatively unstable impurities which normally over a period of time decreasingly exag gerate the results of the subsequent analysis for significant hydrocarbon contenti7 treating said gasesVattein removal from the soil but before analysis, with KMnOe, said treatment being carried 10 4out under such conditions that said iinpurities are largely removed from the gases. protecting the treated gases pending analysis from contamination by other hydrocarbon-containing gases and analyzing the treated gases for significant hydrocarbon content for an indication of petrov liferous deposits .below the locations from which said soil samples were obtained.

EDWIN E. Roms. 

